1. Field of the Invention
This invention relates to electron emission devices. More specifically, this invention relates to the structure and manufacture of electron emissive elements used in flat panel displays.
2. Background Art
In a flat panel display, a matrix of electron emitters emit electrons that impinge on a transparent display panel coated with light emitting material such as phosphor. The principles of a flat panel display can be more clearly explained by referring to FIGS. 1A, 1B, and 1C (collectively FIG. 1), which illustrate a flat panel display structure.
In FIG. 1A, backplate 120 is provided as a support to which electrically conductive emitter layer 113 is attached. Generally conical electron emitters 116 are formed on emitter layer 113. In FIG. 1B, electron emitters 116 are formed within gate holes 115B, under gate layer 115A. Gate layer 115A is separated from emitter layer 113 by dielectric layer 117. Display panel 118 having light emissive layer 110 and anode layer 111 is situated above, and spaced vertically apart from, gate layer 115A.
Portions of gate layer 115A are provided with sufficiently greater voltage than emitter layer 113 and electron emitters 116 to enable layer 115A to extract electrons from electron emitters 116. Anode layer 111 is at a considerably greater voltage than emitter layer 113 or gate layer 116. As a result, a large fraction of the electrons emitted from electron emitters 116 are attracted by anode layer 111 toward transparent panel 118. With anode layer 111 being quite thin, the electrons pass through anode layer 111 and impinge on the phosphor coating 110 on panel 118, causing light emissive layer 110 to emit light.
FIG. 1C shows a cathode structure 100 for a flat panel display. Emitter layer 113 is divided into mutually insulated emitter rows 114, while gate layer 115A is divided into mutually insulated columns 184. For a black and white display, the overlapping area of a row 114 and a column 184 (see FIG. 1D) represents a pixel, the smallest element of a picture. For a color display, several (normally three) overlapping row/column areas form a pixel. In order to cause a selected group of emitters 116 to emit electrons thereby to energize a pixel, an appropriate electric field must be created between electron emitters 116 and gate layer 115A. In particular, a voltage must be applied between a selected row 114 and a selected column 184 to place that row 114 at a suitably greater potential than that column 184, thereby causing electron emission from emitters 116 at that row/column intersection. When the voltage between the selected row 114 and the selected column 184 is below a non-zero threshold value, emitters 116 at the row/column intersection do not emit electrons, and the corresponding pixel is not excited.
Referring to FIG. 1C, a complete picture requires the scanning of every row and every column. In order to have the picture appear to be continuous to the human eye, the scanning must be performed at high speed. Thus the voltage between a specific row and column must change in a very short time.
The geometry of rows 114 and columns 184 together with the thickness H and dielectric constant of dielectric layer 117 determines the crossover capacitance between a row 114 and a column 184. When thickness H is small, the crossover capacitance is large. This capacitance substantially slows down the activation of electron emitters 116, resulting in poor display. Therefore, it is desirable that dielectric layer 117 be thick. When the thickness of dielectric layer 117 increases, the height of electron emitters 116 normally must also increase in order to bring their tips sufficiently close to gate layer 115A to enable layer 115A to extract electrons from them.
A thick dielectric layer also reduces the possibility of short circuiting. During display operation, undesirable conductive paths may be produced through dielectric layer 117 so as to short circuit emitter layer 113 and gate layer 115A. As thickness H (FIG. 1D) of dielectric layer 117 increases, the likelihood of short circuiting gate layer 115A to emitter layer 113 by creating such a conductive path decreases. Further, in FIG. 1A, hollow spaces 119 keep gate layer 115A spaced apart from electron emitters 116. Because gate holes 115B are typically quite small, as little as 80 nm in diameter, a metal particle falling into hollow space 119 may cause short circuiting between gate layer 115A and electron emitters 116. With a thick dielectric layer 117, hollow space 119 would have an elongated profile. A particle falling into hollow space 119 tends to rest within the hollow space and away from gate hole 115B, and thus is less likely to cause short circuiting.
For conical electron emitters with a given aspect ratio (height to base diameter), larger gate holes 115B are required in order to create higher conical electron emitters 116. However, for fine quality picture, it is desirable to have more electron emitters per unit area. Thus it is desirable to have small gate holes. Small gate holes also give greater field strength at the emitters, resulting in lower applied voltage between rows and columns to achieve a given emission current. High aspect ratio cones allow a thick dielectric layer to be used, thus giving the advantages of reduced cross-over capacitance and greater short protection. Consequently, a higher aspect ratio is desirable for making a better cathode structure.
Certain materials such as nickel can be used to create electron emitters with a high aspect ratio. However, nickel does not have other properties desired for electron emitters. For example, nickel has poor chemical robustness. Also, nickel is easily oxidized. Oxidized nickel emitters have an increased extraction voltage and decreased emission stability.
Nickel has a relatively high work function. Work function is defined as the level of energy necessary to energize an electron to such a level that the electron is emitted from the material. A high work function means that a stronger electric field is required between the electron emitter 116 and corresponding column 184 of gate layer 115A in order to energize the electrons. This stronger electric field translates to a greater column-to-row extraction voltage. A high column-to-row extraction voltage is undesirable because it results in high power consumption and more expensive circuitry.
It is therefore desirable to have electron emitters with a high aspect ratio, good chemical robustness and low work function.
In accordance with the present invention, improved electron emitters are provided with high aspect ratios, good chemical robustness and low work function. Electron emitters are formed with electrically non-insulating material that allows deposition to a high aspect ratio at low deposition temperature. One candidate material for the electron emitters is nickel. Electron emitters so made are coated with surface material that has good chemical robustness and low work function. One candidate for the surface material is carbon. The emitter and surface materials may also be chosen for other desirable electrical or chemical properties. Work function of coated emitters is typically reduced by about 0.8 to 1.0 eV.